Question

Give simple chemical tests to distinguish between the following pairs of compounds.
a.) Propanal and Propanone
b.) Acetophenone and Benzophenone
c.) Phenol and Benzoic acid
d.) Benzoic acid and Ethyl benzoate
e.) Pentan-2-one and Pentan-3-one
f.) Benzaldehyde and Acetophenone
g.) Ethanal and Propanal

Answer 1

Hint: The basis of distinguishing between the compounds is its functional group. Tests such as Tollen’s test, Fehling’s test, etc. are used for distinguishing between aldehydes and ketones.

Complete step by step answer:
(i) Propanal and Propanone
Tollen’s test – Test for distinguishing between aldehyde and ketone.
Aldehydes give a positive test – forms a silver mirror. Ketones give a negative test.
Therefore, Propanal gives a positive test. Propanone gives a negative test.

(ii) Acetophenone and Benzophenone
Iodoform test – Methyl ketones give a positive iodoform test.
Acetophenone is a methyl ketone, whereas benzophenone is a phenyl ketone.
Therefore, acetophenone gives a positive iodoform test by giving a yellow precipitate of iodoform with an alkaline solution of iodine. Whereas, benzophenone gives a negative test.

(iii) Phenol and Benzoic acid
Ferric chloride test – phenols give a violet coloration with this test.
Phenol reacts with neutral ferric chloride to form an iron-phenol complex, which gives a violet coloration.
Whereas, benzoic acid reacts with neutral ferric chloride to give a buff coloured precipitate of ferric benzoate.

(iv) Benzoic acid and Ethyl benzoate
Sodium bicarbonate test – Acids give a positive test.
Benzoic acid, being an acid gives a positive sodium bicarbonate test. It gives an effervescence of carbon dioxide gas upon reaction with sodium hydrogen carbonate.
Whereas, ethyl benzoate does not react with sodium hydrogen carbonate and gives a negative sodium bicarbonate test.

(v) Pentan-2-one and Pentan-3-one
Iodoform test – Methyl ketones give a positive iodoform test.
Pentan-2-one is a methyl ketone. Therefore, it gives a positive iodoform test by giving a yellow precipitate of iodoform with an alkaline solution of iodine.
Pentan-3-one is an ethyl ketone and therefore, gives a negative iodoform test.

(vi) Benzaldehyde and Acetophenone
Tollen’s test – aldehydes give a positive Tollen’s test.
Benzaldehyde is an aldehyde. Therefore, it reduces Tollen’s reagent to give a reddish-brown precipitate.
Acetophenone is a ketone and therefore gives a negative test.

(vii) Ethanal and Propanal
Iodoform test – Methyl ketones give a positive iodoform test.
Ethanal has one methyl group linked to the carbonyl carbon atom and therefore, responds to this test.
Propanal does not respond to this test.

Note: Fehling’s Test is another test used to distinguish between an aldehyde and ketone. Aldehyde gives a positive test, whereas ketones do not. For example –
Propanal being an aldehyde reduces Fehling's solution to form a red-brown precipitate of cuprous oxide.
$$C{H}_{3}C{H}_{2}CHO+2C{u}^{2+}+5O{H}^{-}\to C{H}_{3}C{H}_{2}CO{O}^{-}+C{u}_{2}O\downarrow +3H_{2}O$$